Method of controlling temperature distribution of a spacecraft

ABSTRACT

A method of controlling the temperature distribution of a spacecraft is provided which comprises forming on at least a portion of the external surface of the spacecraft a coating comprising metal flakes embedded in a polymeric binder.

United States Patent 1151 3,653,942

Boebel et al. [451' Apr. 4, 1972 54] METHOD OF CONTROLLING 2,531,54111/1950 Spicer ..117/31 ux TEMPERATURE DISTRIBUTION A 3,176,933 4/1965Clemmons ..244 1 S C C T 3,129,703 4/1964 Tabor ..126/270 2,530,21711/1950 Bain ....117/31 UX [72] lnventors: Carl P. Boebel, Tipp City;Gary E. Steven- 2,280,135 4/1942 Ward ..1 17/31 X son, Xenia, both ofOhio 3,409,247 1 1/ 1968 Pezdirtz ..244/1 3 351504 11/1967 De Hart..[73] Assignee. The United States of America as represented by theSecretary of the Air 3428473 2/1969 Langley Force 3,548,930 12/1970 Byrd..244/1 X 3,384,324 /1968 OSullivan ..165/133 X [22] Filed: Apr. 28,1970 PP N 321770 Primary Examiner-William D. Martin AssistantExaminerl-larry J. Gwinnell 52 11.5. C1 ..117 33.3, 117 132 B, 117/137,MING-Harry Herbert, and Cedric Kuhn 117/132 BE, 117/132 BS, 117/132 BF,117/160 R,

' 165/133, 244 SS 7 ABSTRACT [51] Int. Cl ..B44d l/36, B64c 1/40 [5 1[58] Field of Search ..117/137,31, 132,333, R; A method of controllingthe temperature distribution of a 161/171; 244/121, 133, 1 SS; 165/133;126/270 spacecraft is provided which comprises forming on at least a 1portion of the external surface of the spacecraft a coating [56]References Cited comprising metal flakes embedded in a polymeric binder.

UNITED STATES PATENTS 6 Claims, No Drawings 2,366,850 1/1945 Gardner..117/31 UX METHOD OF CONTROLLING TEMPERATURE DISTRIBUTION OF ASPACECRAFI This invention relates to a method of controlling thetemperature distribution of a spacecraft. In one aspect it relates to asubstrate having a coating thereon that has a high solar absorptance toinfrared emittance ratio.

The ability of a spacecraft to meet its design mission is inseparablylinked with maintaining the temperature of its contents within therelatively narrow design limits in which they operate satisfactorily.The controlling element which determines the temperature of the vehicleis the ability of its external surface to exchange energy with itsenvironment. The thermophysical properties of importance in controllingthe surface temperature are (1) solar absorptance (01,), the fraction ofincidentsolar energy which a surface absorbs, and (2) the infraredemittance (c the fraction of heat that a surface radiates compared tothat which a blackbody would radiate at the same temperature.

One method of thermal control involves the concept of providing a solarabsorber surface. Such surfaces absorb solar energy while emitting onlya small percentage of the infrared energy. Thus, their (I /E ratios maybe greater than unity whichmakes it possible to maintain surfaces athigh temperatures while the craft is traveling through space. This is ofgreat importance since it is necessary to maintain certain types ofequipment associated with a spacecraft at a high temperature in order toensure satisfactory operation.

In the past, solar absorber surfaces have been obtained by the use ofpolished metals, chemical conversion coatings, and thin film techniques,such as vacuumevaporation, sputtering, electron beam, and thermaldecomposition of organo-metallic compounds. Another method involvespattern painting where the a,/e ratio can be varied by adjusting thepercent of surface coverage by one coating system with a given set ofoptical properties over another coating system with vastly differentoptical properties. These techniques to obtain high a,/e ratios arelimited from the standpoints of ease of application, especially tocomplex geometrical surfaces, maintenance and repairability, cost, andweight contribution.

The limitations listed in the preceding paragraph could be overcome oralleviated by the development of a suitable coating that could beapplied as a paint. Paints containing a binder filled with aluminumpowder having an a,/e ratio of 1.0 to 1.2 have been used for severalyears for satellite applications. The principal problems associated withmetal pigmented paints with respect to obtaining higher a,/e, ratioshave been their low a, values (0.25-0.40) and the fact that thepolymeric binders absorb infrared energy, thereby causing high emittancevalues (040+). Commercially available paints containing metal particlesother than aluminum, such as copper or bronze, may have high solarabsorptance values, but their formulation is such that they do not havethe necessary low emittance.

It is an object of this invention, therefore, to provide a method forforming a coating on a spacecraft surface that has a high aJs ratio.

Another object of the invention is to provide low cost surface coatingsfor spacecraft that can be readily repaired and maintained.

Other and further objects and advantages of the invention will becomeapparent to those skilled in the art upon consideration of the followingdisclosure.

The present invention resides in a method for controlling thetemperature distribution of a spacecraft which comprises (1) applying toa surface of the spacecraft a solution of a polymeric binder in asolvent for the binder, the solution having dispersed therein flakes ofa metal or alloy, (2) evaporating the solvent from the solution, and (3)curing the polymeric binder with the flakes embedded therein. Uponapplication of the solution, the flakes float on top of the thin layerof the solution. As the solvent evaporates and curing of the binderpolymeric binder covers the layer. The coatings prepared by this methodare characterized by having high (XJGIR ratios.

It is usually preferred to use flakes of copper, iron, nickel, stainlesssteel, bronze or brass. The flakes, which are available from commercialsources, generally vary somewhat in size and are often irregular inshape. The flakes that are preferred for use are those having a diameteror width less than 150 microns, desirably in the range of 40 to 100microns. The thickness of the flakes also varies, but it is usually inthe range of about 0.25 to 2.5 microns.

The formulation, i.e., the polymer solution having metal or alloy flakesdispersed therein, that is applied to a surface of the spacecraft can beconveniently prepared by dissolving the polymeric binder in a solventfor the, binder. Thereafter, the metal or alloy flakes are added to thesolution. Prior to use the solution is stirred to ensure that the flakesare dispersed throughout the solution. A preferred mixing methodinvolves adding the materials to a cylindrical glass container, closingthe container with a Teflon cap, and then placing the container on atwo-rollmill. Rotation of the rolls causes the container to revolve,thereby dispersing the flakes in the solution.

In general, any of the well-known resins, including those used in thepaint art, can be employed as the polymeric binder in preparing'theformulation. Examples of suitable binders include phenolic resins, suchas phenol-formaldehyde, resorcinol-formaldehyde and phenol-furfuralresins; epoxy resins, such as bisphenol A-epichlorohydrin resin;epoxy-novolak resins; polyorganosiloxanes, such as polymethylsiloxaneand polyphenylsiloxane; acrylic resins, such as polyacrylonitrile andpolymers of methyl methacrylate; alkyd resins, such as those preparedfrom phthalic anhydride, maleic anhydride or fumaric acid, and apolyhydric alcohol, such as glycerol, pentaerythritol or sorbitol;nitrocellulose plastics; terephthalate polyesters, such as polyethyleneglycol terephthalate; vinyl polymers, such as polyvinyl chloride,polyvinyl acetate, polyvinyl aldehyde, and copolymers of vinyl chlorideand vinyl acetate; and the like. It is to be understood that mixtures ofresins can be utilized as the polymeric binder.

A number of solvents can be used in preparing the formulation of thisinvention. The solvent selected will depend upon the particular resinused as the binder, and such a selection is well within the skill of theart. Examples of suitable solvents include aliphatic hydrocarbons, such:as pentane, hexane, and isooctane; cycloaliphatic hydrocarbons, such ascyclopentane and cyclohexane; aromatic hydrocarbons, such as benzene.toluene, and xylenes; alcohols, such as methanol, ethanol and butane];ketones, such as acetone, methyl ethyl ketone, and methyl isobutylketone; acetates, such as vinyl acetate, Cellosolve acetate; and thelike.

The amount of each component in the formulation can vary within ratherbroad ranges. Thus, the formulation can contain on a weight basis 3 topercent metal or alloy flakes, l to 20 percent polymeric binder and 19to percent solvent. To obtain the highest (I ratios, it has been foundthat the amount by weight of the flakes must be greater than the amountby weight of the binder, preferably about 1.5 to 4.5 times greater. Byusing the higher proportions of flakes as compared to the binder, aminimum of the binder in the coating prepared by the method of thisinvention is exposed to infrared radiation. As a result the binder issubstantially prevented from absorbing infrared energy which wouldotherwise increase the emittance values and concomitantly lower the 11/6 values.

The formulation can be applied to the surface or portion of the surfaceof the spacecraft by brushing or spraying. However, it is preferred tospray the formulation since a more even coating is obtained andlocations difficult to reach are more preciably shortened by conductingthe steps at an elevated temperature, e.g., from about 50 to l50 C. Itis also within the purview of the invention to add a curing agent to theformulation. The actual curing agent used will depend upon theparticular polymeric binder utilized in the formulation. For example,organotin compounds, such as tin octoate, can be employed as curingagents when a polyorganosiloxane is used as the binder, As anotherexample, amine curing agents, e.g., dior polyfunctional amines, can beused if the binder is an epoxy or an epoxy-novolak resin. The selectionof a suitable curing agent for a particular polymer can be readily madeby those skilled in the art.

As mentioned hereinbefore, the method of this invention makes itpossible to provide coatings having high a /e ratios. In general, theratios vary from about 1.30 to 2.20 depending upon several factors,including the particular metal or alloy flake and polymeric binder usedand the weight ratio of flakes to binder. The method of this inventionwith its capability of forming coatings having a,/e ratios in theindicated range makes it possible to control the temperaturedistribution of a spacecraft. Thus, the exchange of energy between theexternal surface of the spacecraft, or a desired portion thereof, withits environment can be controlled so as to maintain contents of thespacecraft within desired temperature limits.

A more complete understanding of the invention can be obtained byreferring to the following illustrative examples which are not intended,however, to be unduly limitative of the invention.

EXAMPLE I A series of five coating formulations was prepared bydissolving equal amounts by weight of a polyorganosiloxane andpolymethylmethacrylate in xylene. To five cylindrical containerscontaining the solution, there was added copper flakes. The flakes hadan average size of about 70 microns. The flakes were thoroughlydispersed in the solution by placing the capped containers on a two-rollroll mill and rotating them for an extended period of time. Thefollowing are the compositions of the formulation contained in thecontainers:

WEIGHT PERCENT A B C D E Binder 0.96 4.2 7.1 2.0 1.4 Copper flakes 3.816.7 28.6 40.0 42.8 Xylene 95.2 79.1 64.3 58.0 55.7 Totals: 99.96 100.0100.0 100.0 99.9

"Blend of 50 wt 7% polyorganosiloxane (General Electric SR82) and 50 wt72 polymethylmethacrylate (Rohm and Haas 8-66).

The formulations were sprayed on aluminum alloy panels. Evaporation ofthe solvent and curing of the polymeric binder were allowed to proceedat room temperature. Thereafter, the a, and e values of the coatingswere determined and the a,/e

The data in the foregoing table show that high a,,/e, ratios areobtained in accordance with the method of this invention when usingformulations containing a wide range of percentages of copper flakes andbinder. However, highest values were obtained when the weight ratio ofcopper flakes to binder was about 4 to 1.

EXAMPLE [I Formulations were prepared according to the procedure ofExample I in which various metal and alloy flakes were used. The flakeshad an average size of about 70 microns. The following are thecompositions of the formulations:

WEIGHT PERCENT Binder 3.6

Metal or alloy flakes 5.6

Lacquer thinnel 90.8

Total: I000 l) Nitrocellulose.

(2) 45 wt umyl acetate. 5 wt 1 Cellosolvc acetate, 5 wt 7( toluene. and45 wt 1 methyl isobutyl ketonc.

Coatings were prepared by spraying the formulations on aluminum alloypanels after which evaporation of the solvent (lacquer thinner) andcuring of the binder were allowed to proceed at room temperature.Thereafter, properties of the coatings were determined as in Example I.The metal or alloy flakes used in the formulations as well as theproperties of the coatings are shown below in Table 11.

The data in Table II demonstrate that the method of this invention iseffective in producing high tau/e ratios, i.e., substantially greaterthan one, when using formulations containing different metals or analloy in the form of flakes.

In the foregoing examples, solar absorptance ((1,) was determined by useof a dual beam spectrophotometer with an absolute reflectanceintegrating sphere using a xenon energy source throughout the wavelengthrange. Calculation of the a, of each coating involves a procedure ofaveraging the percent reflectance and percent line values over awavelength increment, then multiplying the reflectance by the percentsolar intensity corresponding to this wavelength increment as defined byF. S. Johnson, Satellite Environment Handbook," Stanford Press (1961).These values of reflected energy are summed up for each increment (atotal of 25 points are used) to obtain the integrated solar reflectance,which is subtracted from 100 percent to obtain the solar absorptance.Magnesium oxide is used as the integrating sphere coating, but becauseof the sphere geometry and the dual beam of the spectrophotometer, anabsolute reflectance value is obtained.

The total normal spectral emittance is calculated from 3 to 25 micronsspectral reflectance data from a double beam spectrophotometer equippedwith a hohlraum heated cavity. The reflectance values are corrected forboth the zero and 100 percent datum lines. The 100 percent datum line isobtained using nickel oxide as the reference. The total emittance valueof a given sample is calculated from the reflectance measurements andcompared to a theoretical blackbody at a 75 F. surface temperature.

As will be evident to those skilled in the art, various modifications ofthis invention can be made or followed in the light of the foregoingdisclosure and discussion without departing from the spirit or scope ofthe invention.

We claim:

1. A method of controlling the temperature distribution of a spacecraftwhich comprises:

a. applying as a paint to a surface of said spacecraft a solution of apolymeric binder in a solvent therefor, said solution having dispersedtherein flakes of a metal or alloy selected from the group consisting ofcopper, iron, nickel, stainless steel, bronze and brass and saidsolution comprising, on a weight basis, 1 to percent polymeric binder,19 to 95 percent solvent, and 3 to 80 percent metal or alloy flakes, theamount by weight of said flakes being greater than the amount by weightof said binder;

b. evaporating said solvent from said solution; and

. curing said polymeric binder with said flakes embedded therein toprovide on said surface a coating having a solar absorptance to infraredemittance ratio varying from about 1.30 to 2.20.

2. A method according to claim 1 in which said solution is applied tosaid surface by spraying thereon.

3. A method according to claim 1 in which the weight ratio of saidflakes to said binder is in the range of about l .5 to 4.5.

4. A method according to claim 1 in which said polymeric binder isselected from the group consisting of phenolic resins, epoxy resins,epoxy-novolak resins, poly organosiloxanes, acrylic resins, alkydresins, nitrocellulose plastics, terephthalate polyesters, and vinylpolymers; and said solvent is selected from the group consisting ofhydrocarbons, alcohols, ketones and acetates.

5. A spacecraft with at least a portion of its external surface havingadhered thereto a coating having a solar absorptance to infraredemittance ratio varying from about 1.30 to 2.20. said coating comprisingflakes of a metal or alloy selected from the group consisting of copper,iron, nickel. stainless steel, bronze and brass, and a cured polymericbinder, the weight ratio of flakes to binder being about 1.5 to 4.5,said flakes being embedded in said binder contiguous to one another soas to form a substantially continuous layer of flakes on said binder,and a thin film of said binder covering said layer.

6. The spacecraft according to claim 5 in which said coating consistsessentially of said flakes and a polymeric binder selected from thegroup consisting of phenolic resins, epoxy resins, epoxy-novolak resins,polyorganosiloxanes, acrylic resins, alkyd resins, nitrocelluloseplastics, terephthalate polyesters, and vinyl polymers.

2. A method according to claim 1 in which said solution is applied tosaid surface by spraying thereon.
 3. A method according to claim 1 inwhich the weight ratio of said flakes to said binder is in the range ofabout 1.5 to 4.5.
 4. A method according to claim 1 in which saidpolymeric binder is selected from the group consisting of phenolicresins, epoxy resins, epoxy-novolak resins, poly organosiloxanes,acrylic resins, alkyd resins, nitrocellulose plastics, terephthalatepolyesters, and vinyl polymers; and said solvent is selected from thegroup consisting of hydrocarbons, alcohols, ketones and acetates.
 5. Aspacecraft with at least a portion of its external surface havingadhered thereto a coating having a solar absorptance to infraredemittance ratio varying from about 1.30 to 2.20, said coating comprisingflakes of a metal or alloy selected from the group consisting of copper,iron, nickel, stainless steel, bronze and brass, and a cured polymericbinder, the weight ratio of flakes to binder being about 1.5 to 4.5,said flakes beIng embedded in said binder contiguous to one another soas to form a substantially continuous layer of flakes on said binder,and a thin film of said binder covering said layer.
 6. The spacecraftaccording to claim 5 in which said coating consists essentially of saidflakes and a polymeric binder selected from the group consisting ofphenolic resins, epoxy resins, epoxy-novolak resins,polyorganosiloxanes, acrylic resins, alkyd resins, nitrocelluloseplastics, terephthalate polyesters, and vinyl polymers.